Tissue perfusion is a measure of the amount (volume) of blood passing through a unit quantity of the tissue and is often measured with the unit ml blood/100 g tissue. Since all blood tissues are at the same time being supplied with nutrients and excrete waste products through diffusion between tissue cells and the blood, tissue perfusion is a very important factor indicating the state of health of a tissue. A method for the measurement of tissue perfusion is therefore highly pertinent, for instance for monitoring tissue during and after surgical operations and transplantations. Monitoring of potentially threatened tissue, e.g. muscular tissue, whose blood supply may become adversely affected by increasing pressure in the connective tissue membrane of the muscle, would be highly pertinent as an indication of when a pressure relieving operation should be initiated. Likewise monitoring of internal perfusion caused by the formation of oedemas in a heart stopped during operation could provide valuable information about the need of external supply of nutrients to the tissue of the heart. Within medical research, perfusion is an important parameter too.
A number of methods for determination of tissue perfusion are known. A technique consisting of an injection into the relevant tissue of radioactive xenon as a tracer and measuring the decay of radioactivity as a function of time has been described (see Larsen et al., 1966. Blood Flow through Human Adipose Tissue Determined with Radioactive Xenon. Acta physiol. scand. 66, pp 337–345), but this technique suffers from a number of drawbacks in that its temporal resolution only amounts to approximately half an hour which is insufficient in many situations. Furthermore the location of the injection of the radioactive matter into the tissue relative to the location where the radioactivity is being measured is not particularly well-defined and finally, the application of radioactive matter per se involves potential hazards.
Another method of measuring tissue perfusion utilises continuous injection of ethanol during microdialysis. During microdialysis a fluid is being pumped very slowly through a fibre inserted into the tissue of the patient. The concentration of the fluid is in equilibrium with the surrounding tissue as the catheter is diffusion-open and the fluid is being collected via a return fibre. This method also suffers from an insufficient temporal resolution.
WO 97/46853 discloses a method and a microsensor which is able to measure tissue perfusion. The sensor comprises a tracer-permeable insert placed in a mouth of a tracer reservoir confined by a container, whereby said insert forms a permeable wall portion of the reservoir. A sensoric tip of a transducer is placed inside the insert or immediately outside of the latter. From the specification as a whole it appears that the tip of the transducer is very small, a diameter of 2 μm being mentioned. Consequently the transducer detects or measures the tracer concentration or pressure in a single point or in an extremely limited area. This also applies, if the transducer is provided with an inner cylindrical cavity, which is closed by the permeable insert or by a separate membrane forming the end wall of the transducer.
In connection with monitoring tissue perfusion for instance during surgical operations, the above-mentioned prior art suffers from the drawbacks of either insufficient temporal resolution or a very limited measurement space.